The present invention relates to a Zeeman atomic absorption spectrophotometer.
An atomic absorption spectrophotometer is known as an apparatus for use in quantitatively analyzing metal elements contained in sewage, food, urine or the like. Among the atomic absorption spectrophotometers, one using the Zeeman effect is known as an apparatus which permits the analyzing of a very small quantity of metal and which has a high sensitivity. By the use of the Zeeman effect, the detecting limit is improved to a higher level by one or two orders of magnitude than that of an ordinary prior art atomic absorption spectrophotometer. Zeeman atomic absorption spectrophotometry has recently been studied by many scientists and has been observed to be a highly reliable and promising technique.
Still there is a major problem in that Zeeman atomic absorption spectrophotometry has a draw-back phenomenon in its calibration curve. This draw-back phenomenon will be explained with reference to FIG. 1.
FIG. 1 shows an example of the calibration curve of cadmium, which is measured by the use of the conventional Zeeman atomic absorption spectrophotometer, as described in H. Koizumi et al, Analytical Chemistry, Vol. 49, No. 8, p. 1106 (1977). This spectrophotometer is so constructed that a magnetic field intersecting an optical axis at a right angle is impressed upon a sample atomizing unit and that a rotating polarizing element is interposed between a light source and the sample atomizing unit. This produces a pair of polarized light components which are, respectively, perpendicular to and parallel to the magnetic field. The parallel component is absorbed by the vaporized sample but the perpendicular component is not. Thus, the difference in absorption between the two components gives a measurement of the absorption of the sample with a correction for background absorption. A calibration curve for the material being analyzed is drawn by the use of the peak value of the difference between the beams having the polarized components which have passed through the polarizing element and which intersect each other at a right angle.
As is apparent from FIG. 1, the difference absorption is gradually increased with the increase in concentration. As is also apparent from FIG. 1, the curvature of the calibration curve becomes higher as the concentration increases. Moreover, the calibration curve has a peak in the vicinity of 40 ppb and exhibits a dropping phenomenon in the higher concentration range. This dropping phenomenon is the draw-back phenomenon of the calibration curve.
Since the calibration curve becomes a two-valued function in the high concentration range because of the draw-back phenomenon, it is impossible to accomplish the measurement. In the example shown in FIG. 1, moreover, the curvature of the calibration curve is greater than 25 ppb, for example, so that sufficient accuracy cannot be attained in the actual analysis.